ASP.NET AJAX in Action by Alessandro Gallo, David Barkol, and Rama Krishna Vavilala, is one of the best books that I've seen covering AJAX from the Microsoft perspective.  Their book is wonderful at giving the reader the exact details that they need to know about how to take advantage of Microsoft's AJAX technologies on both the client and the server, as well as getting the client and the server to work together.

When you think of Microsoft AJAX, most developers would probably think first of the UpdatePanel control in ASP.NET.  Using UpdatePanel is easy.  The developer drops it onto a page, adds one or more ASP.NET server controls as content for the UpdatePanel, and instantaneously those nasty page refreshes go away.

What most developers, in my opinion, are not thinking about when they hear the term Microsoft AJAX is the rich client-side library written in JavaScript that allows developers to do so much more on the client side.  The authors recognize this problem and spend significant time explaining the richness of the JavaScript environment before introducing the concept of the UpdatePanel control.  The end result of this book is that the developer has started to master the client-side development aspects of developing with JavaScript and calling web services on the server by the time the developer starts looking into the server-side aspects of AJAX.

Special attention in the second half of the book is placed on the component and control model of the Microsoft AJAX library.  Developers will learn how to create new non-visual JavaScript components, behavior extenders in JavaScript, and new JavaScript-based HTML controls.  Once the client-side aspects of these components and controls have been covered, the authors then explain and demonstrate how to wrap your new JavaScript components in ASP.NET server controls to enable the drag-and-drop designer experience.

This book is easy to read.  I found very few errors in it (none that I can actually recall).  Overall, this book really helped me to better understand and advance my use of Microsoft's AJAX technologies and pushed me into spending a lot more time learning how to develop in JavaScript on the browser, something which I admittedly was not very good at.  I definitely recommend buying this book the next time that you're at Borders.